Article pubs.acs.org/accounts
Molecular Rotors Built in Porous Materials Angiolina Comotti, Silvia Bracco, and Piero Sozzani* Department of Materials Science, University of Milano Bicocca, Via R. Cozzi 55, 20125 Milan, Italy
CONSPECTUS: Molecules and materials can show dynamic structures in which the dominant mechanism is rotary motion. The single mobile elements are defined as “molecular rotors” and exhibit special properties (compared with their static counterparts), being able in perspective to greatly modulate the dielectric response and form the basis for molecular motors that are designed with the idea of making molecules perform a useful mechanical function. The construction of ordered rotary elements into a solid is a necessary feature for such design, because it enables the alignment of rotors and the fine-tuning of their steric and dipolar interactions. Crystal surfaces or bulk crystals are the most suitable to adapt rotors in 2D or 3D arrangements and engineer juxtaposition of the rotors in an ordered way. Nevertheless, it is only in recent times that materials showing porosity and remarkably low density have undergone tremendous development. The characteristics of large free volume combine well with the virtually unhindered motion of the molecular rotors built into their structure. Indeed, the molecular rotors are used as struts in porous covalent and supramolecular architectures, spanning both hybrid and fully organic materials. The modularity of the approach renders possible a variety of rotor geometrical arrangements in both robust frameworks stable up to 850 K and self-assembled molecular materials. A nanosecond (fast dynamics) motional regime can be achieved at temperatures lower than 240 K, enabling rotor arrays operating in the solid state even at low temperatures. Furthermore, in nanoporous materials, molecular rotors can interact with the diffusing chemical species, be they liquids, vapors, or gases. Through this chemical intervention, rotor speed can be modulated at will, enabling a new generation of rotor-containing materials sensitive to guests. In principle, an applied electric field can be the stimulus for chemical release from porous materials. The effort needed to obtain strong dipoles that are noncentrosymmetrically mounted onto rotors and do not hamper rotational motion is a further aspect of this research activity. Thus, materials showing dielectric properties in response to applied electric fields have been fabricated. This may lead to challenging materials that are promptly responsive to an applied electric field, altering the ferroelectric or antiferroelectric ground state by fast dipole reorientation when subjected to electric polarization.
1. INTRODUCTION Motional phenomena occurring in the solid state have been frequently envisaged as a way to conjugate the dimensional stability and mechanical properties of solid materials with the flexibility and dynamics of fluids. Descriptions have already been given of several solid materials endowed with intrinsic dynamics, wherein some components of the structure possess enough freedom to undergo fast, thermally activated motion.1−3 In addition to the example of soft materials, such as cross-linked polymers above their glass transition, even shape-persistent structures, such as crystals, can show remarkable dynamics in their various components. Indeed, hard matter can contain mobile elements and flexible molecular moieties, which can be designed to induce new and interesting properties. The efforts of engineering these molecular elements, in both a rational fashion and possibly a concerted way, has © XXXX American Chemical Society
enabled the construction of molecular motors and molecular devices.4−6 In this context, molecular elements that perform simple motion trajectories and, in particular, rotary motion (molecular rotors) play a major role in performing special functions: for instance, dipoles mounted on molecular rotors may lead to switchable organic ferroelectric materials. Actually, tunable dielectrics are obtained if rotors are ordered in 2D or 3D arrays. Concerted rotor dynamics of dipoles in such regular environments can generate slow wave propagation in the radiofrequency regime, similar to acoustic waves, and can be used to build radio frequency filters. Therefore, dipole−dipole interactions in anisotropic arrangements such as ordered solids and crystalline-like aggregation states open challenging Received: May 5, 2016
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Figure 1. Representation of periodic mesoporous organosilicas and the various linkers.
perspectives in signal processing. In other applications, the switching of molecular rotation takes place in association with a change of spin state, birefringence, or electric conductivity.7−10 Molecular dynamics in solids is correlated to the presence of available space, sometimes called f ree volume, which allows motion to occur in a low-density material. This empty room typically accounts for a few percentile of the total volume, but it allows the dynamical moieties to achieve enough freedom. This principle is well-documented in amorphous materials such as polymers above their glass transition and hexagonal rotator phases of linear alkanes.3,11,12 Thus, low density is a prerequisite for the design of mobile elements in solids, and this principle has been exploited in a variety of materials, the stimulus being to obtain new molecular mobility benchmarks.2 A major breakthrough was achieved when a remarkable amount of free volume, and consequently of molecular freedom, was provided by permanently porous solids. Indeed, some materials were engineered to generate low-density architectures in which large excess space was tailored to form galleries or channels, exposing mobile elements to an empty space. Thus, the highly porous materials of the last generation constitute a playground for enhancing dynamics in the solid state.13−15 Such challenging systems can “breath” through the pores by guest adsorption/desorption, and the entire population of rotors interacts with molecular species diffusing in the material. Such direct interaction has been exploited to regulate rotor speed at will thanks to the specific nature of the molecules diffused in the nanopores, building up a pervasive and tunable interface. Furthermore, porous solids can integrate into their 3D structures a second component that can behave as a dynamic substructure. This is the case of guest molecular rotors assembled in an orderly fashion in host−guest binary compounds.3 This versatile method allows the realization of guest rotary motion without the formation of covalent bonds between the rotary axis and the framework, although the control over the motional trajectories is sometimes problematic. Collectively, the combination of two properties in a same material, that is, fast rotors and the existence of structural pores that offer large accessible porosity, feeds the field of molecular rotors and motors with exuberant perspectives still to be explored. They include coherent dynamics in oriented systems and the opportunity for intervening through the accessible pores on performing solid-state reactions, which enable the construction of sophisticated machinery. The anisotropic reorientation and rotor alignment allows for macroscopic
manipulation of vectorial properties, such as dielectric phenomena. Rotor-containing porous materials have been realized by selfassembly of suitable organic or hybrid building blocks following a few strategies, which involve covalent bonds and metal− organic or supramolecular interactions that sustain their architectures. They will be described in the following, starting from the prominent group of hybrid materials and metal− organic frameworks (MOFs) and later shifting to fully organic covalent and supramolecular networks.
2. ROTORS IN HYBRID MATERIALS 2.1. Mesoporous Organosilicas
Periodic mesoporous organosilicas (PMOs) consist of fully covalent architectures and conjugate the thermal robustness of siloxane bonds, typical of silica, with the chemical stability of carbon−carbon organic bonds.16 They generally belong to the family of mesoporous materials since they exhibit cylindrical pores of more than 2 nm, obtained by condensation of organic−inorganic building blocks on the scaffold of templating micelles. The reaction of bis-trialkoxyorganosilanes leads to the formation of three siloxane bonds at each molecular end, thus composing a cross-linked porous structure constituted by a large part of organic, especially aromatic, matter (Figure 1). The self-assembly process not only allows the generation of a single network of alternated organic and inorganic nanolayers, but also, under suitable conditions, the precise organization of periodically aligned monomer units in the framework. XRD clearly indicates two hierarchical levels of order, both on the molecular scale and on the nanometer scale. While the latter derives from the hexagonal arrangement of the channellike pores, the molecular order demonstrates that the organic linkers are set in a favorable manner to engineer molecular rotors in ordered arrays. The molecular order is compatible with even rather elongated organic linkers, for instance, aromatic units substituted in para positions or diphenyls, which exhibit pivotal bonds lying on a rotational axis. Other favorable features for the construction of rotors is the distance at which the rotational axes are set; in fact, the siloxane bonds act as spacers to keep the rotors apart one from the other. Molecular rotors in mesoporous organosilicas were first described in 2008 in diphenylsilica, which, at that time, was the prototype example of rotors in porous materials (Figure 2).17 The discovery was based on the collection of both 1H and 13C spin−lattice NMR relaxation times (T1) at variable temperature, these being most sensitive to motional phenomena over a B
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The data can be fitted by the Kubo−Tomita equation, which describes the modulation of relaxation by carbon−hydrogen vector reorientation.18 13C(T1) relaxation times recorded at 75 MHz showed a maximum at 310 K, indicating that at this temperature the rotors respond in the 107 Hz range. 1H(T1) relaxation times at 300 and 30 MHz confirm that the sample response falls in this motional regime with interconversion energy barriers as low as 6.4 kcal/mol. This result stimulated the study of other p-phenylene rotorcontaining periodically ordered PMOs. A direct proof of the motional mechanism can only be derived by an invaluable method such as 2H spin−echo NMR.19 It requires the preparation of deuterated samples with C−D bonds in specific positions on the rotary part. If reorientational frequencies k are slower than 103 Hz as occurs in rigid structures, the C−D bond cannot move significantly; thus a typical Pake-pattern spectrum with a singularity spacing of d = 125−136 kHz for aliphatic or aromatic carbons is detected. Faster motional regimes produce signal averaging and articulated profiles, which can be simulated by anisotropic motional models. Deuterated para-phenylenesilica showed 2H NMR differentiated patterns as a function of temperature: starting from the “static” (Pake pattern) spectrum detected at low temperature, a dramatic evolution of the spectrum profile occurs on increasing temperature, the signal intensity increasing toward the central part of the spectrum, which is an indication of motional averaging (Figure 3).20 Intermediate motional regimes give rise to profiles with multiple singularities and a complex shape. The simulation of the patterns according to the trajectories of the C−D bonds yielded rotor speed at each particular temperature. The spectra show singularities separated by 33.4 Hz with the shape of a restricted Pake spectrum of d/4, which can be interpreted by
Figure 2. (a) Temperature-dependent 1H relaxation rates at 30 and 300 MHz and 13C relaxation rates at 75 MHz (for CH carbon) for mesoporous diphenylsilica. (b) Schematic picture of rod-like diphenyl rotors vertically aligned along the channel axis.
wide frequency range 103−108 Hz. The maximum relaxation rate occurs when thermal agitation frequencies match the observation frequency, according to the formula τ × ω ≅ 1.
Figure 3. 2H NMR spectra of d4-p-phenylenesilica compared with the simulated spectra for (A) empty nanochannels and (B) nanochannels filled by octadecyltrimethylammonium bromide (experimental spectra on the left and calculated spectra on the right). Exchange rates k (Hz) are reported. (C) Arrhenius plot of the two compounds. Adapted with permission from ref 20. Copywright (2010) John Wiley & Sons. C
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Accounts of Chemical Research two-site 180° flip reorientation of the phenyl rings about their axes. The rate of the rotors reaches values as high as 108 Hz at room temperature, but the thermal robustness of the carbon− carbon, carbon−silicon, and silicon−oxygen covalent bonds allowed observations of dynamic moieties at considerably high temperature with substantial mechanism stability. From the Arrhenius plot, we can deduce a remarkably low activation energy of 6.2 kcal/mol and attempt frequency of 2 × 1012 Hz, in agreement with the small p-phenylene rotational inertial mass.2,21 The direct NMR demonstration of p-phenylene−organosilica nanochannel accessibility, detected by hyperpolarized Xe-gas diffused in to the channels,22 prompted us to perform a totally new experiment: actually, the open porosity of the material offered the perspective of regulating rotary motion by guest stimuli. Indeed, when p-phenylenesilica channels were filled with guest molecules such as octadecyltrimethylammonium bromide, a drastic reduction of the rotational rates by 3 orders of magnitude was observed and the activation energy increased up to 10 kcal/mol. The energy barrier increment between guest-filled and empty channels resulting in 4 kcal/mol: such measurement allowed to estimate, in an unusual way, the weak intermolecular interactions occurring at the host−guest extended interfaces. Modulation of rotor dynamics was obtained by a variety of guests, including H2O (not presented), showing for the first time the possibility to tune the rotor speed in porous materials (Figure 4).
Figure 5. Schematic representation of the divinyl-p-phenylenesilica wall and its 2D-glass transition. Adapted with permission from ref 23. Copywright (2012) ACS.
become extremely fast. At higher temperature, there occur much wider librations manifested by the apparent weakening of the spectral shoulders. The transition, involving some gain of motional freedom, is associated with an increase in heat capacity, as observed in differential scanning calorimetry trace showing the appearance of a glass transition. 2.2. Rotors Connected by Metal Nodes in Metal−Organic Frameworks
MOFs or porous coordination polymers (PCPs) are one of the most attractive areas in current research because they hold promises for gas capture, molecular separation, catalysis, polymerization, and optical or electronic properties. The term “dynamics” in MOF structures regards intrinsic framework flexibility based on switching between two differently porous architectures or is used to indicate the insertion of mobile moieties in the fixed reference system of the framework. In the latter case, rotors have been found in a few families of MOFs. In a cadmium-based PCP containing pyrazine linkers pivoted on the metal node, the specific configuration ensures an extremely low barrier (approximately 2 kcal/mol) for the foursite rotation about the metal−organic bonds placed on the same axis (Figure 6).24 Nevertheless, the low attempt frequency of 106 Hz limits the spinning speed to moderate motional regimes of 105 Hz at room temperature. Furthermore, MOF dicarboxylate aromatic ligands, like terephthalate, display Csp2−
Figure 4. 2H NMR spectra of mesoporous p-phenylenesilica with (b) C20, (c) tetraethylammonium, and (d) octadecylammonium compared with the empty matrix (a). The exchange rates (Hz) are reported. Adapted with permission from ref 20. Copywright (2010) John Wiley & Sons.
Periodic order has been maintained in mesoporous organosilicas constructed with a major amount of organic component and rotor-containing elongated linkers. In the particular case of divinyl-p-phenylenesilica, the rotor lies between two carbon− carbon double bonds that set the rotors at a distance from the inorganic layers (Figure 5). This geometrical arrangement results in the organic elements gaining a great degree of freedom: the compound deuterated on the phenyl ring shows a rapid C−D bond reorientation in the megahertz regime at room temperature, but more interestingly, a transition about room temperature takes place:23 in fact, an abrupt rotational dynamics change, clearly non-Arrhenius behavior, speeds up the motional regime to
Figure 6. Crystal structure of the Cd-based MOF: (a) the pillared layer structure; (b) pyrazine rotor. Reproduced with permission from ref 24. Copywright (2006) John Wiley & Sons. D
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Accounts of Chemical Research Csp2 pivotal bonds with a partial conjugation to the aromatic electronic system and a high activation energy for rotation, 10− 15 kcal/mol, rendering the flipping motion sufficiently fast only at high temperatures (450 K).2,25,26 The choice of para-phenylene as the basic rotary unit was the most attractive for its low molecular mass: it was discovered that the additional organic functions on the phenylene ring are not necessarily a burden that hampers rotation, but they may be beneficial to the flip motion since the lateral substituent destabilizes the ground state. Moreover, the introduction of functionalities carrying dipoles into rotators brought about additional benefits. In the case of IRMOF-2, the linker is the Br-terephthalate group with the Zn metal node. Dielectric response was observed in the audio frequency regime and found to follow a hopping model of molecular dipole relaxation, with a single energy barrier of 7.3 kcal mol−1.27 The behavior of IRMOF-3, which contains the aminic linker, showed by 1H NMR relaxation, allowed a decrease in the energy barrier down to 5 kcal/mol although the increased molecular mass slowed the motion with an attempt frequency of 109 Hz and reorentational rates of 106 Hz at 365 K.28 The rather constrained local environment for the phenylene rings in 1D pore systems of MIL-47(V) and MIL-53(Cr), compared with a 3D system such as MOF-5, was reflected in the variable temperature 2H NMR experiments.25 Although the spectra are becoming rather distorted by the effect of paramagnetic centers in close proximity, it has been possible to determine the activation energy of 10−11 kcal/mol.
3. ROTORS IN POROUS MOLECULAR CRYSTALS 3.1. Molecular Porous Materials Figure 7. (a) Crystal structure of porous compound and (b) experimental (colored line) and (c) simulated (black line) 2H NMR spectra as a function of temperature; θ (deg) corresponds to the semiamplitude of libration; below T = 200 K librations are neglected. The reorentational rates in Hz are reported. The energy barrier of rotor speed accounted for 4.7 kcal/mol. (d) 4,4-Bis(sulfophenylethynyl)benzene linker showing the central p-phenylene rotor. Adapted with permission from ref 35. Copywright (2014) ACS.
Porous molecular materials offer stimulating perspectives in building crystalline architectures by exploiting the versatility of the organic synthesis. Only a few were described until recent years.29−32 This is likely due to the objective difficulties in preventing the tendency of molecules to aggregate into a closepacked crystal, thus canceling any significant porosity. In fact, the absence of intermolecular interactions as strong and directional as covalent bonds is manifested by the tendency to structural collapse. Indeed, until recent years, the principle by which any molecular system could reach its maximum close packing was considered inviolable. However, structural stability can be notably increased if molecular rigidity and appropriate shape factors of the molecules reduce favorable packing.33,34 Porous architectures can be induced by host−guest formation followed by mild guest removal. This process results in crystals with permanent pores, which are generally accessible from the gas phase. Hydrogen-bonding and Coulombic interactions may contribute to stabilize low-density frameworks. Indeed, rod-like molecules containing aromatic rings are most suitable to engineer rotors appended in their core, like a wheel mounted on an axel. Rotors based on p-phenylene moieties and lying between two triple carbon−carbon bonds produce a rotational energy barrier lower than 1 kcal/mol.35 Triple bonds extend the length of the rotational axis, while a further extension of the molecular axis can be achieved by additional p-phenylene units at the end of molecule, as depicted in Figure 7. The rod-like molecules bearing sulfonate groups at their ends self-assemble with alkylammonium cations, forming a porous architecture held together by charged-assisted hydrogen bonds. 13 C Spin−lattice relaxation times thoroughly describe the motional behavior of each molecular fragment: the two
peripheral p-phenylene rings show, in the examined temperature range, relatively long relaxation times on the order of 20− 40 s, indicating that they are far from being interested by rotation. In contrast, the central ring finds a most efficient relaxation with T1 relaxation times as short as 0.25 s, which, at the applied magnetic field of 7 T, is diagnostic of rotary motions with correlation times in the nanosecond regime. Contrarily, carbon nuclei lying on the molecular axes are substantially unaffected by motion; in fact they display considerably long relaxation times. These observations are fully consistent with 2H NMR spectra at variable temperature, which depict C−D bonds of the labeled central ring rapidly reorienting about the rotation axis. This mechanism persists in a range extended down to very low temperatures, showing extremely fast spinning speed such as 107 Hz even in the 200− 240 K range. Moreover, the central ring is fully exposed to adjacent channels and may be sensitive to gases flowing-in. The material was exposed to I2 vapors, and already at very low iodine pressure, the rotor dynamics was affected (Figure 8). Actually, the energy barrier for rotation was drastically increased up to 10.5 kcal/mol. The rotor dynamics could be switched off and E
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systems are clathrates, wherein rotary motion cannot be systematically engineered. Porous materials provide suitable predetermined room for guests to be included, as shown by the versatility of such systems in controlling the anisotropic motion of numerous diverse guests. Elongated and polymer guests find their perfect complement in host crystalline nanochannels: this prerogative was recognized several years ago and gave rise to fast spinning of the guest chains about their main axes. Long segments of the polymer chains underwent diffusional rotation even for high molecular mass polymers. Hence, low molecular mass hydrocarbon chains displayed rotational behavior in quasi-cylindrical crystalline channels.36−42 In fact, tight channels promote fast dynamics, not restrict motion, compared with much larger and comfortable channels wherein flexible polymer chains relax to intricate conformations that may convert into each other in a concerted and complex way, thus gaining space occupation without extended chain-segment spinning. This indicates that complementary host-to-guest conditions play a key role in promoting rotary motion, and in particular, a circular crosssection is compatible with the rotational motion. The spinning of extended polymer chains in nanochannels mimics the rotator phases observed at high pressure and temperature in bulk polymers and hydrocarbons and enabled elucidation of the motional state of polymers in the bulk phase, which is of importance to understand heat exchange and frictional behavior.11 An extension of the above concepts leads to the formation of channel inclusion compounds with suitable elongated guests.43,44 Such guests could be engineered with end segments capping the rotary moiety, which is appended to the central part of the molecule: thiophene oligomers showed ultrafast mobility when covalently connected to two alkoxy chain-ends. Ultrafast motion regimes were achieved, as MAS NMR and laser assisted Fourier transform ESR put in evidence. The inclusion-compound approach was later exploited in an innovative way to produce molecular rotors anchored on a crystal surface. The guest molecules were constructed with
Figure 8. Structure and 2H NMR spectra of empty (left) and iodineloaded (right) compound (experimental, black and violet lines; calculated, gray lines) at 295 K. The exchange rates are reported. Adapted with permission from ref 35. Copywright (2014) ACS.
on by I2 absorption/desorption, showing remarkable response of material dynamics to the interaction with volatile species, suggesting the use of molecular crystals in sensing and pollutant management. 3.2. Rotary Motion of Guests in Host−Guest Systems
The vast majority of guest molecules, when included in solid hosts, find a way to gain at least some degree of motion. The host dictates the symmetry and the anisotropic shape of the cavities, and in response, the included guests increase their motion, occupying the overall space available by multiple and dynamically explored conformations. Guest diffusive motion is commonly observed in inclusion compounds, but such motion may not generate molecular rotors since the rotation does not necessarily occur about a single axis. In fact, many host−guest
Figure 9. (a) Two-dimensional 1H−13C HETCOR NMR spectra of surface inclusion compound at distinct contact-times. (b) Dipolar-rotor-bearing molecules inserted into tris-o-phenylenedioxycyclotriphosphazene (TPP) nanochannels. Adapted with permission from ref 45. Copywright (2012) ACS. F
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Figure 10. (a) Structure of PAFs. (b) Rotational rates of empty and C12- and C20-loaded d4-PAF3 vs temperature. The energy barrier of rotor speed in empty d4-PAF-3 accounted for 4.4 kcal/mol. (c) Large-amplitude p-phenylene librations occurring at high temperature along with (d) 2H NMR spectra of d4-PAF-3 (experimental, black lines, and calculated, blue lines). The spectra were simulated by applying reorentational rates k > 108 Hz on both two-site 180° flip reorientation and libration mechanisms. Adapted with permission from ref 47. Copywright (2014) John Wiley & Sons.
inclusion-compound approach and porous-host/rotary-guest materials, engineered to realize molecular rotors in solids. A few interesting examples of spinning guests showed energy barriers favorable for rotation.46
specialized moieties, each of them designed for a different purpose.45 Typically, they are individually constituted by rotary heads (rotators), bulky stoppers (which prevent the rotators from entering the channels) and long hydrocarbon tails, which fit perfectly in the channels and are rooted in the porous crystal by multiple weak interactions (Figure 9). The channel geometry ensures that the rotators protrude from the crystal surface and are set at a distance by the hexagonal arrangement of the host lattice, thus, promoting the motion of the polar heads, which are free to reorient. The juxtaposition of rotor molecules has been established unconventionally by 2D MAS NMR spectra, showing a double correlation with 13C and 1H nuclei. The necessary condition to collect a 2D signal is the close-space proximity, independently of the covalent links, between the interacting nuclei. Therefore, the alkyl tails of the guest inserted in the matrix must produce host−guest strong correlation signals, while the signals for stopper at the end of the matrix channels appear under more severe recording conditions (longer contact-times). Importantly, the absence of correlation signals between the rotator and the host proves that the rotating heads are far from the bulk crystal. A number of publications followed the above seminal papers, giving rise to further examples of applications of TPP for the
4. ROTORS IN POROUS ORGANIC FRAMEWORKS A recent and stimulating research subject is that of porous covalent frameworks, which exhibit surface areas as large as 4000−7000 m2/g and robust structures stable up to 450 °C. They are mostly constituted by rigid rod-like organic struts and display voids of about 15 Å, thus belonging to the category of microporous materials. The first and up-to-date example of molecular rotors in porous organic frameworks (POFs) was described in the case of so-called porous aromatic frameworks (PAFs).15 They are substantially constituted by aromatic rings anchored covalently onto tetrahedral nodes, which can be carbon, silicon, or germanium (PAF-1, PAF-3, and PAF-4, respectively). The full exposure of the struts toward empty cavities provides them with tremendous freedom, limited only by the torsion barrier at the pivotal bonds and the exposure to species visiting the voids through the open and light architecture. Given the above premises, porous organic frameworks are perhaps the most promising and versatile materials in this field. In a first example (PAF-3), the rotor G
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Accounts of Chemical Research speed has been measured to be 106 Hz already in a cold environment below 200 K (Figure 10). This value is obtained by 2H NMR: the powder lineshape fits perfectly a fast two-site 180° flip reorientation of the p-phenylene units.47 Furthermore, the above-mentioned robustness of the scaffold, which is sustained by a barely deformable 3D-network allowed study of the rotor dynamics at far higher temperature than in any other organic system. This provided a unique opportunity to detect motional mechanisms under such extremely severe conditions. Actually, the way phenylene rings rotate changes appreciably in the highest temperature range. The experimental profile displays a narrower line truncated by a flat terrace on the top. This peculiar profile corresponds to a calculated spectrum consistent with a large displacement (plus/minus 37°) away from the 0° and 180° arrangements, where the limit positions of librations are much more frequently explored than the intermediate ones like in a pendulum model. Thus, four sites are populated during the rotary trajectory with extremely low interconversion barrier, and free rotation was then approached at 550 K. Such fast-spinning behavior was slowed immediately after iodine vapor diffusion to the porous material. This experiment proved the high porosity by an unconventional spectroscopic detection and that the molecular entities of the material can exist in two distinct, dynamic and static, forms. The effect by which dynamics can be regained reversibly was achieved on command after a short vacuum application. N-alkanes such as C12 and C20 linear hydrocarbons diffused into the nanochannels from the melt phase also act as brakes, which hamper the p-phenylene rotation at sufficiently low temperature. This is due to the CH/π interactions installed between the guest and the molecular rotors in the host. In fact, the energy barrier is increased in the amount of 2−3 kcal/mol. On increasing temperature, the rotors undergo a motional transition and switch back to regimes as fast as in the pure matrix, because the energy of thermal agitation competes with the weak CH/π interactions between the two components.
Figure 11. Dielectric loss vs temperature of FOS-1 (a) and FOS-2 (b) at 140, 1400, and 14000 Hz. Dotted lines are nonfluorinated mesoporous material. Adapted with permission from ref 48. Copywright (2015) John Wiley & Sons.
highlighted an intense dipole response at variable frequency and temperature. The maximum dielectric losses become evident when the applied electric frequency matches the molecular dipole dynamics. Under these conditions, dielectric response, typical of a fast dipole reorientation, is obtained by the stimuli of an applied electric field. Furthermore, the mesochannels are open and accessible to gas molecule diffusion, and rotor-mobility could be individually regulated by I2 vapors. The iodine enters the channels of the periodic structure and reacts with the pivotal double bonds of the divinyl-fluoro-phenylene rotors, affecting their motion and the dielectric properties. This constitutes an intriguing example of solid state reaction affecting massively both the molecular dynamics and the macroscopic properties of a material.
5. DIPOLAR ROTORS IN FLUORINATED MESOPOROUS ORGANOSILICAS The ultimate goal of the field of rotors in porous materials is the realization of rotating dipoles by the insertion of polar groups on the molecular rotor. It is not sufficient that the dipole distribution is non-centrosymmmetric, but it is necessary that dipole oscillations would produce molecular dipole displacement. Therefore, the principles that enable a rotor to behave as a rotating dipole imply a precise molecular design. In this respect, hybrid organosiloxane bricks are very versatile, because they can be synthesized with a variety of substituted aromatic groups and self-assemble into an organized mesoporous crystal, as depicted above. In order to design dipolar rotors, the molecular dipole moment must not align with the rotation axis, and dipoles should be generated by low bulkiness atoms. Thus, the appropriate substitution of one or two hydrogen atoms of a p-phenylene ring with a fluorine atom generated dipoles that can rotate, even if the inertial mass and the lateral encumbrance would be slightly larger. These achievements have been recently accomplished in fluorinated mesoporous organosilicas (FOSs, Figure 11). Indeed, two new compounds, divinyl-monofluoro- and difluoro-p-phenylenesilicas, denominated FOS-1 and FOS-2, were prepared and showed high surface areas and excellent adsorption behavior toward carbon dioxide.48 Dipole dynamics was studied by dielectric relaxation measurements that
6. PERSPECTIVES Molecular motion in solid materials is a fascinating phenomenon, especially if a quite rigid and robust framework coexists with dynamics. Here, we discussed our contribution and that from other groups regarding this aspect, in conjunction with the feature of some solids of being porous and accessible from the gas phase. An additional step forward has been achieved when dielectric properties and dipole dynamics were realized. By this way, a few prerogatives were put together in an unconventional manner. Pioneering work demonstrates the feasibility of rotor-dynamics control by the action of chemical species absorbed from the vapor or gas phase, which intervene directly on the rotor dynamics, with the challenging perspective of building selective sensors and devices for release of chemicals on command. Moreover, the robustness of the covalent frameworks cohabits, paradoxically, with the motional behavior of the rotating elements and considerably extends the working temperature range.
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The manuscript was written through contributions of P.S., A.C., and S.B. Funding
PRIN-MIUR and Cariplo Foundation are acknowledged for financial support. Notes
The authors declare no competing financial interest. Biographies Angiolina Comotti received her Ph.D. in Chemistry at the University of Milan and is currently Associate Professor. She was visiting scientist at ETHZ and NYU. Her main activity is focused on supramolecular chemistry and porous materials with particular interests in structural and dynamical aspects. Silvia Bracco received her Ph.D. in Materials Science. She was a postdoctoral fellow at UCSB and is currently Assistant Professor. Her activity deals with porous materials and solid state NMR. Piero Sozzani is full professor of Macromolecular Chemistry at University of Milano_Bicocca. He has been visiting scientist and consultant at AT&T Bell Laboratories. His main research interests encompass the study of nanostructured materials, polymerization in confined spaces, and solid state NMR spectroscopy.
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